Abstract
Spar platforms are treated as cost-effective and resourceful type of offshore structure in deep water. With increasing depth there are significant changes in its structural behaviour due to coupling of spar hull-mooring line along with radical influence of mooring line damping. So these phenomena should be precisely counted for accurate motion analysis of spar mooring system. In present study, spar platform are configured as a single fully coupled integrated model in ABAQUS/AQUA. Non-linear dynamic analysis in time domain is performed adopting Newmark-β automatic time incrementation technique. Non-linearities due to geometric, loading and boundary conditions are duly considered. Displacement and rotational responses of spar and mooring tensions are obtained during long-duration storm. spar responses get significantly modified and mean position of oscillations gets shifted after longer wave loading. The surge, heave and pitch responses are predominantly excited respectively. The energy contents of PSDs of these responses reduce considerably after long wave loading. Mooring tension responses are significantly different reflecting the damping effect of mooring lines. The pitch response is fairly sensitive to the wave loading duration. After long duration of storm the wave frequency response increases. However, low frequency and wave frequency responses may simultaneously occur due to synchronising sea states.
Highlights
Exploration and development of offshore oil and gas in shallow and intermediate water depths has traditionally been carried out using the conventional jacket type fixed platforms
Coupled dynamic behaviours of hull/mooring/ riser elements of spar platform have correspondingly been investigated by some other researchers (Chen et al 2006; Jameel, Ahmad 2011; Kim et al 2001a, b, 2005)
Chen and Zhang (1999) presented the response of a spar constrained by slack mooring lines to steep ocean waves by two different schemes: a quasi-static approach (SMACOS), and a coupled dynamic approach (COUPLE) to reveal the coupling effects between a spar and its mooring system
Summary
Exploration and development of offshore oil and gas in shallow and intermediate water depths has traditionally been carried out using the conventional jacket type fixed platforms. Coupled dynamic behaviours of hull/mooring/ riser elements of spar platform have correspondingly been investigated by some other researchers (Chen et al.2006; Jameel, Ahmad 2011; Kim et al 2001a, b, 2005). Ran et al (1999) studied coupled dynamic analysis of a moored spar in random waves and currents They followed the solution in time-domain and frequency-domain analysis. The mooring line dynamics takes into account the instantaneous tension fluctuation and damping forces with time-wise variation of other properties These forces are active concurrently on spar hull cylinder. It is not needed to match the force, displacement, velocity and acceleration at the fairlead location iteratively The output from such analyses is horizontal, vertical and rotational motions of platform and mooring line responses. The structural response behaviour in steady state after 2000 and 8000 sec. of wave hitting have been extracted in the form of surge, heave and pitch motion
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